1. Field of the Invention
The present invention relates to a nonreciprocal circuit device such as an isolator or a circulator used in a high-frequency band such as the microwave band, and to a communication device using the same.
2. Description of the Related Art
In recent years, with mobile communication devices speeding toward miniaturization, a further reduction in the size of nonreciprocal circuit devices used in these devices is strongly required. In order to realize this size-reduction, Japanese Unexamined Patent Application Publication No. 11-97908 has disclosed a nonreciprocal circuit device using a magnetic body having a rectangular parallelepiped shape. When using such a magnetic body having a rectangular parallelepiped shape, one center conductor is disposed parallel with one side of the magnetic body, the two other center conductors are disposed so as to tilt toward each side, and all center conductors are arranged so as to intersect one another at an angle of substantially 120 degrees, in a state of being electrically insulated from one another. Herein, the conductor widths and the inter-conductor spacings of these center conductors used are each set to be the same.
Typically, the impedance of an input/output portion in the circuit where these nonreciprocal circuit devices are used has a predetermined value (usually 50 Ω), and the impedance at each port in the nonreciprocal circuit device (hereinafter, referred to as a “port impedance”) is also set to be a predetermined value.
The three center conductors which are disposed on the rectangular-parallelepiped shaped magnetic body so as to tilt toward each side of the magnetic body at an angle of 120 degrees, form a rotationally asymmetric configuration. Hence, when the conductor widths and the inter-conductor spacings of the center conductors are each made to be the same, the port impedance of the center conductor which is parallel with one side of the magnetic body becomes higher than that of each of the two other central conductors. For example, in the above-described conventional nonreciprocal circuit device, when the port impedance of the two center conductors which is disposed so as to tilt toward each side of the magnetic body is set to be 50 Ω, the port impedance of the center conductor which is disposed parallel with one side of the magnetic body can become 80 Ω. This is, the two center conductors which is disposed so as to tilt toward each side of the magnetic body have a symmetric configuration with respect to the magnetic body, while the center conductor which is disposed parallel with one side of the magnetic body has a configuration asymmetric with respect to the two other center conductors. In the above-described conventional nonreciprocal circuit device, therefore, a problem occurs that the reflection characteristics of the port of the center conductor disposed parallel with one side of the magnetic body deteriorates.